I regularly edit 20 mp pictures on my i7 with 8 GB. Speed is fine. It's not instantaneous. When I do Lightroom with a browser with multiple tabs, the system does bog down. I've seen over 4 GB of usage just for Lightroom and Edge as the major programs (even though a bunch of things are running in my system tray).
If you're only doing Lightroom with absolutely nothing else running, you should be fine. But if this machine is a serious machine with a serious workflow, you really should consider at least 8 GB.